[ ] — Vector / Array notation

[A B C] is a vector of the items A, B and C, whatever their number. Inside brackets, spaces separate items, and a line break means ⋄. Items can be subjects or functions, and they are evaluated left to right. [Y] is a one-item vector, and [] is the empty vector ⍬. A strand of literals needs no brackets: 1 2 3 is [1 2 3].

a←1 ⋄ b←2
[a+b a×b]          ⍝ 3 2
≢[+/ ÷ ≢]          ⍝ 3ₓ
[5]≡,5             ⍝ $t
[]≡⍬               ⍝ $t

[A;B C] separates items with ;. Inside each item, spaces separate runs, as they do outside brackets.

[1 2;3 4]          ⍝ [[1 2] [3 4]]
x←2 4 9
[+/x ; +/x ÷ ≢x]   ⍝ 15 5

[A⋄B] is an array whose major cells are A and B, padded with fill to a common shape. [A⋄] has the single major cell A. A row that is one number is a cell of rank 0.

[1 2⋄3]          ⍝ [1 2⋄3 0]
⍴[1 2 3⋄]         ⍝ [1 3]ₓ
[1⋄2]            ⍝ 1 2
⍴[[1]⋄[2]]       ⍝ [2 1]ₓ

In […]ₓ, every number is exact, at any depth. Each item must be a literal, and each number must be whole.

[1 0 1]ₓ≡1ₓ 0ₓ 1ₓ  ⍝ $t

A bracket list with any key:value item is one keyed vector. An item without a key is a position with no key.

["aa":1 "bb":2]              ⍝ "aa" "bb":1 2
["a":1 "b":(•ucs 65 66)]     ⍝ "a" "b":[1 "AB"]

As an assignment target, [a b]←Y destructures. Brackets don’t index. Index ⌷ selects. Types of brackets compares brackets with parentheses and braces.

Errors

• SYNTAX: a trailing ;, or both ; and ⋄ in one pair of brackets